Dexron (no "t") III is sold under various labels such as D/M, D3 compatible, etc, because Dexron III is no longer a current license. I'd just pick a quality name-brand and use it.
If you're asking if Dex VI is backwards-compatible, it is for many applications but not all.
What vehicle do you have.? If you insist on Dex 3 , here is some synthetic ATF meeting Dex 3 at Walmart. Packaging might be different for you so read the back of the container. Hope that helps.
Just a suggestion, when asking question like this, please state the vehicle make and model and what application in that vehicle is of interest (Trans or Power Steering, etc.).